1. Field of the Invention
The present invention relates to a phase shift mask and a method for repairing a defect of a phase shift mask, and in particular, it is concerned with a phase shift mask and a method for repairing a defect of a phase shift mask in which a defective portion of a phase shifter is generated in the vicinity of a boundary between a light transmitting portion and a phase shifter portion.
2. Description of the Background Art
Recently, high integration and miniaturization have been greatly developed in semiconductor integrated circuits. Accordingly, miniaturization of circuit patterns formed on a semiconductor substrate has been developed rapidly.
A photolithography technique is, among others, well known in the art as a basic technique for pattern formation, for which various developments and improvements have been made. However, there is still an increasing need for miniaturization of a pattern, and thus, there is a stronger need for improvement in resolution of a pattern.
In general, a resolution limit R (nm) in the photolithography technique using a demagnification exposure method is expressed as EQU R=k.sub.1 .multidot..lambda./(NA) (1)
where .lambda. represents a wavelength (nm) of light, NA represents a numerical aperture of a lens, and k.sub.1 is a constant depending on a resist process.
As can be seen from the above expression, in order to improve a resolution limit, values of k.sub.1 and .lambda. are made smaller and a value of NA is made larger. In other words, it is sufficient to reduce the constant depending on the resist process, with the wavelength being made shorter and NA being increased.
Nevertheless, it is difficult technically to improve a light source or a lens, and there is generated such a problem as to incur degradation of a resolution, because a depth of focus .delta. of light (.delta.=k.sub.2 .multidot..lambda./(NA).sup.2) is made shallower by shortening the wavelength and increasing NA.
Now, with reference to FIG. 24, a description will be made for a cross section of a conventional photomask, an electric field on the photomask, a light intensity on the resist film, and a pattern transferred onto the resist film.
First, with reference to FIG. 24(a), a structure of a photomask 30 will be described. In photomask 30, a mask pattern 38 having a predetermined shape is formed on a transparent glass substrate 32. This mask pattern 38 includes a light shielding portion 34 formed of chromium or the like and a light transmitting portion 36 exposing transparent glass substrate 32.
With reference to FIG. 24(b), the electric field of exposure light of photomask 30 is provided along the photomask pattern.
With reference to FIG. 24(c), the light intensity on a semiconductor wafer will be described. When a fine pattern is to be transferred, beams of exposure light transmitted through the photomask are intensified by each other due to diffraction and interference in a portion of adjacent pattern images where beams of light are overlapped.
Therefore, a difference in the light intensity on the semiconductor wafer is reduced, so that a resolution is deteriorated. As a result, a pattern transferred on the resist film is, as shown in FIG. 24(d), cannot reflect the photomask pattern accurately.
In order to solve this problem, a phase shift exposure method using a phase shift mask, for example, has been proposed in Japanese Patent Laying-Open Nos. 57-62052 and 58-173744.
Now, with reference to FIG. 25, a phase shift exposure method using a phase shift mask disclosed in Japanese Patent Laying-Open No. 58-173744 will be described.
FIG. 25(a) is a cross sectional view of a phase shift mask 40. FIG. 25(b) shows an electric field on the photomask. FIG. 25(c) shows an amplitude of light on the resist film. FIG. 25(d) shows light intensity on the resist film. FIG. 25(e) shows a pattern transferred onto the resist film.
First, with reference to FIG. 25(a), a structure of phase shift mask 40 will be described. A mask pattern 50 having a predetermined shape is formed on a transparent glass substrate 42. This mask pattern 50 includes a light shielding portion 44 formed of chromium or the like and a light transmitting portion 46 exposing transparent glass substrate 42. At every other light transmitting portion 46 exposing transparent glass substrate 42 is provided a phase shifter portion 48 formed of a transparent insulating film such as a silicon oxide film.
With reference to FIG. 25(b), in the electric field on the photomask formed by beams of light transmitted through phase shift mask 40, the transmitted beams of light have phases inverted alternately by 180.degree.. Therefore, in adjacent pattern images, overlapping beams of exposure light transmitted through phase shift mask 40 have phases inverted from each other.
Accordingly, the amplitude of light on the resist film is provided as shown in FIG. 25(c). As to the light intensity on the resist film, beams of light are canceled with each other due to interference in a portion where beams of light are overlapped, as shown in FIG. 25(d). As a result, there is provided a sufficient difference in the light intensity of exposure light on the resist film allowing improvement of the resolution, so that the pattern along mask pattern 50 can be transferred onto the resist film as shown in FIG. 25(e).
However, although the phase shift exposure method using the above-described phase shift mask is highly effective in a periodic pattern such as lines and spaces, arrangement of phase shifters and the like becomes very difficult in the case of a complex pattern, so that a pattern cannot be set arbitrarily.
As a phase shift mask for solving this problem, a phase shift mask of attenuation type, for example, has been disclosed in JJAP Series 5 Proceedings of 1991 International MicroProcess Conference pp. 3-9 and Japanese Patent Laying-Open No. 4-136854.
A phase shift mask of attenuation type disclosed in Japanese Patent Laying-Open No. 4-136854 will be described below.
FIG. 26(a) shows a sectional structure of a phase shift mask 52 of attenuation type. FIG. 26(b) shows an electric field on the photomask. FIG. 26(c) shows an amplitude of light on the resist film. FIG. 26(d) shows light intensity on the resist film. FIG. 26(e) shows a pattern transferred onto the resist film.
First, with reference to FIG. 26(a), a structure of the phase shift mask of attenuation type will be described. Phase shift mask 52 includes a transparent quartz substrate 54 and a phase shift pattern 64 having a predetermined pattern shape on transparent quartz substrate 54.
Phase shift pattern 64 includes a light transmitting portion 62 exposing the transparent quartz substrate, and a phase shifter portion 60 formed on transparent quartz substrate 54, which has a smaller transmittance of exposure light than in light transmitting portion 62 and converts a phase of exposure light by 180.degree..
Phase shifter portion 60 is formed by a chromium layer 56 having a transmittance of 5-40% with respect to the exposure light transmitted through light transmitting portion 62, and a shifter layer 58 converting a phase of the exposure light by 180.degree..
In the electric field on the photomask of the exposure light transmitted through phase shift mask 52 having the above-described structure, a phase of exposure light is inverted at an edge portion of the exposure pattern, thus providing the amplitude of exposure light on the resist film as shown in FIG. 26(c).
Thus, the light intensity on the resist film is necessarily 0 at the edge portion of exposure pattern, as shown in FIG. 26(d). As a result, there is provided a sufficient difference in the electric field of exposure pattern between light transmitting portion 62 and phase shifter portion 60 so as to obtain a high resolution, whereby the pattern along the phase shift pattern can be transferred onto the resist film as shown in FIG. 26(e).
The applicant of the present invention has developed a phase shift mask for solving a problem in the phase shift mask in Japanese Patent Laying-Open No. 4-335523.
While phase shift mask 52 shown in FIG. 26 includes phase shifter portion 60 having a two-layered structure of chromium layer 56 and shifter layer 58, a phase shift mask 66 shown in FIG. 27 described in Japanese Patent Laying-Open No. 4-335523 includes a phase shifter portion 70 formed of a single material having the same transmittance as that of chromium film 56 and having the same phase angle as in phase shifter portion 60.
More specifically, a phase shift pattern 74 having a predetermined-shaped pattern is formed on a transparent quartz substrate 68 in phase shift mask 66.
Phase shift pattern 74 includes a light transmitting portion 72 exposing transparent quartz substrate 68 and a phase shifter portion 70 formed of a single material. A material of phase shifter portion 70 includes a MoSi nitride oxide film, a MoSi oxide film, a Cr oxide film, a Cr nitride oxide film and a Cr nitride carbide oxide film.
The electric field on the photomask, the amplitude of light on the resist film, light intensity on the resist film, and the transferred pattern on the resist film in the case of using phase shift mask 66 are as shown in FIG. 28(a)-(e). As can be seen from these figures, the same effect can be obtained as in the case of FIG. 26.
Now, a method for repairing a defect when a defect is generated in phase shift pattern 74 of phase shift mask 66 will be described below.
First, with reference to FIG. 29, defects generated in phase shift pattern 74 include a remaining defect (opaque defect) 78 and a pinhole defect (clear defect) 76. In order to inspect such a defect, a die to die inspection system is carried out by using, for example, a light-transmitting pattern defect inspection system (manufactured by KLA, type 239HR). This defect inspection system generally utilizes light emitted from a light source of a mercury lamp.
Through inspection, remaining defect 78 which is a portion of the phase shifter left in a region of phase shift pattern 74 to be etched, and pinhole defect 76 which is a pinhole or a missing portion generated in the phase shifter to be left are detected.
Now, this remaining defect 78 and pinhole defect 76 are repaired. Remaining defect 78 is repaired by a laser blow repairing apparatus using a YAG laser 82, such as is used in the conventional photomask.
Another method for removing the remaining defect is by gas-assist etching using an FIB (Focused Ion Beam).
As to repairing of pinhole defect 76, the pinhole defect is filled with a carbon-type film 80 by FIB assist deposition using an aromatic-type gas, as is generally used in the conventional photomask. The FIB assist deposition can make the process easier and reduce the repairing cost.
Since carbon-type film 80 on the phase shift mask repaired as described above is not peeled off even during cleaning conducted in the later step, a favorable phase shift mask can be obtained.
However, there is a problem in the above-described conventional technique as below.
FIG. 30 is a plan view of a phase shift mask 84 of attenuation type wherein a phase shifter portion 86 and a light transmitting portion 88 are included.
Also, there are provided a pinhole defect 90 generated only in a region of phase shifter portion 86 and a pinhole defect 92 formed in the vicinity of a boundary between light transmitting portion 88 and phase shifter portion 86.
In the following, how these pinhole defects 90 and 92 are repaired will be described. As to pinhole defect 90, carbon-type film 80 is usually used, as shown in FIG. 29.
On the contrary, in the case of repairing of pinhole defect 92, a transmittance of exposure light in this region should be considered.
For example, with reference to FIG. 31, the case where pinhole defect 92 is repaired by filling with carbon-type film 94 by the FIB assist deposition will be discussed.
If exposure is carried out normally by using a phase shift mask including light transmitting portion 88 and phase shifter portion 86 without any pinhole defect 92, then a complete circular pattern 98 is formed in a positive resist film 96 after etching, as shown in FIG. 32. The light intensity of exposure light on the resist film taken along a line A--A and a line B--B are plotted in FIGS. 34 and 35, respectively.
On the contrary, if the phase shift mask in which defects are repaired as shown in FIG. 31 is exposed, an asymmetric pattern 100 is formed in resist film 96, as shown in FIG. 33. The light intensity of the exposure light on the resist film taken along a line C--C and a line D--D are plotted in FIGS. 34 and 35, respectively.
With reference to FIGS. 34 and 35, when the normal resist pattern is formed as shown in FIG. 32, the light intensity on resist film is symmetric about a central axis L, as represented by solid lines a and b. On the contrary, the light intensity on resist film of the pattern shown in FIG. 33 has an oval pattern in which light intensity profile is expanded outwardly as shown by a dotted line c.
The reason for this is that since an opaque carbon-type film is formed in the vicinity of the boundary between the light transmitting portion and the phase shifter portion, such a boundary portion does not serve as a phase shift mask, whereby the same event as is described in photomask 30 shown in FIG. 24 is generated.
Thus, in view of a strong need for miniaturization of a semiconductor apparatus, if a dimensional error is generated in a pattern of the resist film, there are provided disadvantages such as electrical variation in a memory portion of a semiconductor memory device, insufficient contact of bit lines, and short circuit due to insufficient alignment margin.
Also, if a pinhole defect 106 generated in a phase shifter portion 102 of a phase shift pattern formed by lines and spaces of a light transmitting portion 104 and a phase shifter portion 102 is repaired with a carbon-type film 108 as shown in FIG. 36, a portion 114 in which the pattern 110 is thinned is generated as can be seen in pattern 110 of the resist film in FIG. 37.
If an interconnection layer or the like is patterned with such a resist film, the resulting interconnection layer includes a thinner portion. Therefore, disadvantages such as resistance increase and disconnection in that portion are incurred, thus greatly deteriorating the performance of semiconductor device.